Holography is one of the ways to record a three-dimensional image of an object. The original principle of holography was created by Dennis Gabor in 1948 and the first hologram developed by Dennis Gabor is the so-called in-line hologram. However, due to lacking high strength coherent light at that moment when the holography was first developed, the holography was not popular. It was not until laser beam, which is basically an intensive coherent light, was developed in 1960s that the holography has been significantly improved and commercialized.
Generally speaking, holography uses two coherent light beams, such as laser beams, of which one, called reference beam, is directly projected to a photographic film and the other, called object beam, is first projected to an object to be recorded and then reflected to the photographic film by the object. The two beams form an interference pattern on the photographic film and thus record thereon the spatial geometric characteristics of the object and the photographic film with an interference pattern formed thereon called hologram. Sometimes, it may require two or more object beams to photograph a large object which occupies a great space. The interference pattern on the photographic film forms a diffraction grating recording the information of optical wavefront of the object beam reflected from the object.
The holography requires the reference beam and the object beam to be identical coherent light and thus preferably, the two beams are obtained from the same source and split into two beams by a beam splitter.
In forming the hologram, the reference beam and the object beam may both be projected onto the photographic film at the same side thereof and a hologram formed in this way is called a transmission hologram. Alternatively, the reference beam and the object beam may be projected onto the photographic film along substantially opposite directions, namely they are respectively projected onto the two opposite surfaces of the photographic film, and this is the so-called reflection hologram.
To reconstruct the three-dimensional image of the object, a display beam, which is exactly identical to the reference beam, is projected to the hologram along the same path of the reference beam. The display beam passes through the diffraction grating constituted by the interference pattern of the reference beam and the object beam and diffract so as to form a three-dimensional image of the object.
The holography has been widely used in a variety of fields and new applications are still in development. One of the most useful applications of the holography is in the nondestructive inspection field. Certainly, the holography is an excellent tool in optics, for example in the inspection of high precision optical instruments, such as the lens.
It is also possible to use the holography to develop new arts, such as three-dimensional movies and other art applications. However, one of the most commonly known applications of the holography is in the personal identification (PID) document, such as a credit card. As a matter of fact, almost all major credit cards have already adapted hologram as an identification mark. Nevertheless, the very essence of the holography has not been fully exploited, especially in the field of PID. For example, one of the features of the holography is that only is a display beam which is exactly the same as the reference beam used to form a hologram projected onto the hologram along the same path, namely at the same incidental angle relative to the hologram, then a three-dimensional image may be formed at the same distance away from the hologram as the object of which the image is recorded on the hologram is relative to the hologram. By checking the incidental angle and the relative position of the image with respect to the hologram, it is more capable to find out a counterfeit PID that bears a counterfeit hologram figure thereon, for it is difficult to perfectly re-produce a hologram without knowing what kind of coherent light is used to form the hologram and at what angle the reference beam is projected onto the hologram film.
Another potential usage of the holography is the "grating bar code" which comprises a number of sets of parallel and equally-spaced grating extending along different directions which, upon incident of a laser beam, form a number of sets of light spots having different spacing distance determined by the spacing distances of the grating. The spacing distances of the grating encode symbols and the spacing distances of the light spots decode the symbols.